Tone signal generator



April 28, 1970 v R. B. s cHREoNGos-r ETAL 3,509,395

TONE SIGNAL GENERATOR Filed Nov. 25, 1968 3 Sheets-Sheet l April 28, 1970 R. B. scHREcoNGosT ETAL 3,509,395

TONE SIGNAL GENERATOR Filed Nov. 25, 1968 3 Sheets-Sheet 2 wunnlM l Y ve/75ans.'

April 28, 1970 R. B. scHREcoNGosT ET Al- 3,509,395

TONE SIGNALG'ENERATOR Filed Nov. 25, 1968 3 Sheets-Sheet :3

United States Patent O1 hee U.S. Cl. 310-168 12 Claims ABSTRACT OF THE DISCLOSURE A tone signal generator for an electronic organ or similar musical instrument is disclosed. The generator develops a full octave of twelve fundamental notes employing twelve tone wheels each of similar construction, one wheel for each signal frequency. The various wheels are rotated at constant speeds and each forms part of a magnetic circuit which includes a pickup head and coil mounted stationarily adjacent to each wheel. Each wheel comprises a pair of spaced metal discs with scalloped or serrated edges and a disc-shaped plastic permanent magnet sandwiched between the discs. The edges of the metal discs of the wheel are aligned so that the serrations are staggered from one another in relation to a pair of pole pieces of the pickup head. The generator thus may develop tone signals in the pickup coils that are alternating current in nature and can be of a suicient magnitude as to be employed without amplification in successive components of the musical instrument.

The generator is so constructed that identical pickup heads may be employed at each tone wheel despite varying numbers of serrations. Also disclosed is an alternative construction for the pickup head which has the advantage of developing higher amplitude signal outputs.

BACKGROUND OF THE INVENTION Field of the invention The present invention is directed toward a new and improved tone signal generator for electrical tone signal generation for musical instruments.

DESCRIPTION OF THE PRIOR ART One of the basic types of tone signal generating systems used in electric organs and similar musical instruments is illustrated in the patent to Laurens Hammond No. 1,956,350. The arrangement is to provide a synchronous electric motor to drive a plurality of rotating serrated discs, in this art usually referred to as tone wheels, one for each frequency, in close proximity to permanent magnet pole pieces having pickup coils. The tone wheels form portions of magnetic circuits with the pole pieces and thus their rotation generates an alternating current in each pickup coil at the desired frequency. The frequency generated in each coil depends upon motor speed, the gear ratio of the drive from the motor to the particular tone wheel, and the number of serrations around the periphery of the wheel.

In the particular organization of the above-mentioned Hammond patent, there is a tone wheel for each fundamental tone signal required in the instrument and some extra wheels which supply upper harmonic structure above the fundamental limits of the keyboard.

Another system which makes use of rotating wheel type generators is illustrated in the patent application of William B. Ayres, Ser. No. 560,217, filed June 24, 1966. In this system a group of twelve tone wheel generators supplies all of the signals for the top octave only of the desired group of signals, and the signals of lower frequency are obtained from these originally generated signals by means of cascaded frequency divider circuits.

3,509,395 Patented Apr. 28, 1970 A tone signal generator of the same general type as the present invention is also disclosed in the pending application for United States Letters Patent of Lyman l. Haviland and Herbert E. Meinema, entitled Tone Signal Generator, which was iled on Mar. 24, 1967, assigned Ser. No. 625,670 and is assigned to the same assignee as the present invention.

All presently known tone wheel type musical signal generators, including those discussed above, use tone wheels which are comprised of a single wheel or disc for each frequency.

SUMMARY OF THE INVENTION A musical tone signal generator constructed in accordance with the present invention comprises means for rotating at a preselected constant radial velocity, a tone wheel which comprises a pair of serrated edge discs with a permanently magnetic disc sandwiched between the pair of discs and a pickup head mounted adjacent to the edges of the serrated discs from which an alternating electric tone signal of a predetermined fundamental frequency is derived.

Another advantage of the invention is that it readily lends itself to generation of electrical tone signals of superior amplitude, thereby eliminating the need for certain special ampliers ordinarily required.

BRIEF DESCRIPTION OF THE DRAWING The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings, in the several gures of which like reference numerals identify like elements, and in which:

FIG. 1 is a diagrammatic plan view, partly broken away, of a tone signal generator constructed in accordance with the present invention;

FIG. 2 is a fragmentary transverse sectional view of the tone generator of FIG. l in greater detail substantially as seen along the line 2-2 in that figure;

FIG. 3 is an enlarged sectional view of one part, a tone wheel, of the tone signal generator of FIGS. 1 and 2 substantially as seen from the line 3-3 of FIG. 2;

FIG. 4 is a perspective view with parts `broken away of a portion of a simplied tone generator similar in principle to the generator shown in FIGS. 1-3, which is useful in illustrating the #basic operation of the invention; and

FIG. 5 is a fragmentary view of a portion of another generator constructed in accordance with the present invention illustrating an alternate construction of one part thereof, a pickup head assembly.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. l there is depicted a tone signal generator which is generally designated by the numeral 10. The tone signal generator 10 is of the type that may be employed in an electrical musical instrument, such as an electrical organ, as a primary source for the tone signals which are ultimately audibly reproduced. The generator 10 develops twelve different fundamental musical tone signals comprising one octave of the conventional musical scale.

The full octave tone signal generator 10 includes a motor plate or base 11 upon which is mounted a synchronous electric motor 14. Adjacent to the base 11 and to the motor 14 is a tone signal generator module generally indicated by the numeral 16. The module 16 is aliixed to an upstanding planar generally square wall 12 which is affixed to and rises from the base 11. The generator module 16 is made up of two essentially similar submodules 18 and 20. Each of these comprises similar parts interconnected in essentially the same manner. As such, only the submodule 18 will be described in detail it being understood that the construction and operation of the submodule 20 is essentially the same. Also, in general similar numerals will be employed to identify similar parts in both submodules 18 and 20.

Each of the submodules 18 and 20 includes a pair of parallel spaced mounting plates 24 and 26. The two modules 18 and 20 are affixed to the wall 12 by means of four mounting rods or support shafts, such as the shafts 27 which pass perpendicularly through the mounting plates 24 and 26. These shafts 27 are distributed about the wall 12 at its corners and run perpendicularly through the parallel mounting plates 24 and 26 and are affixed to the plates 24 which face the wall 12 of each submodule 18 and 20 by a washer-retaining ring assembly 29. The forward plate 24 of submodule 18 is adjacent to the wall 12 and serves as a convenient mounting place for a capacitor 15 for the synchronous motor 14.

lEach of the plates 24 and 26 is provided with bearings 25 which journal a main drive shaft 28. The shaft 28 may be formed in sections, one section per submodule, which are joined together as by a coupler 31.

This construction allows each submodule such as 18 or 20 to be standardized as much as possible and allows for additional submodules, if desired, to be joined to the generator 10.

lOne end of the drive shaft 28 is connected through a flexible coupling 30 to the shaft of the motor 14. The flexible coupling 30 allows the shaft 28 to be driven at the motor speed, but avoids alignment problems, and, at least to some extent, pulsations in the motor shaft speed are thereby absorbed in the flexible coupling 30.

Six tone wheels 32, 33, 34, 35, 36 and 37 are mounted within the submodule 18, three each upon two shafts 39 and 40 parallel to the drive shaft 28, and are driven through gear trains from the main drive shaft 28. The wheel shafts 39 and 40 are each mounted for rotation at their extreme ends in bearings 23 formed in the plates 24 and 26. The shaft 39 is driven from a gear 41 which is affixed thereto and meshes with a driven gear 42 affixed to the main shaft 28. Similarly the shaft 40 is driven from a gear 43 which meshes with a gear 44 affixed to the main shaft 28. These gear trains 41-42 and 43-44 drive the shafts 39 and 40 at differing angular velocities. The shafts 39 and 40 are frictionally driven respectively by the gears 41 and 43 (which are rotatably mounted thereon), by means of coil springs 38 and 38', respectively, which press the gears into frictional engagement with collars or hubs on the shafts.

It should be here noted that the gear ratios of the second submodule 20 are preferably different from those of thesubmodule 18 so that the wheel shafts of that module are driven at yet different angular velocities from those of the wheel shafts of submodule 18. One preferred relationship of the gearing will be described in more detail later.

The tone wheels 32-37 are freely and rotatably mounted on their respective shafts 39 or 40 and axed thereto by flexible couplings, which together with the flexible coupling or spring 38 which drives the shafts 39 and 40 and the flexible coupling 30, greatly reduce variations in angular speed and minimize problems of alignment.

Mounted outward from each of the tone wheels 32-37 is a pickup head assembly 50. These assemblies 50', which are best shown in FIG. 2, are mounted between a pair of transverse brackets, which extend between the plates 24 and 26. These brackets include a large channel bracket 46 and an L-shaped bracket 47 which are generally aligned parallel to an extension of a radius of the tone wheels 32-37. Two sets of the brackets 46 and 47 are provided on each side of the submodule 18 spaced equal distances above and below the central horizontal plane through the submodule. The upper bracket pair 46-47 on the left side has two pickup head assemblies 50, adjacent to the outer wheels 32 and 34, while the lower bracket pair has a single assembly 50 for the center wheel 33. On the other side of the submodule 18, as shown in FIG. 1, the arrangement is reversed with the assemblies 50 for the outer wheels 35 and 37 being positioned between the lower bracket pair 46-47'and the assembly 50 for the center wheels 36 being held by the upper bracket pair 46-47.

The large channel brackets 46 of both the upper and lower pair of brackets are each positioned to face and be adjacent to the central horizontal plane of the submodule 18. Each of the large channel brackets 46 is held and affixed at each of its extreme ends to the plates 24 and 26 byA means of a nut and bolt assembly 48 whose bolt head passes through one of a pair of outstanding tabs 45 or 49. The tab 45 projects inward from the plate 24 at a right angle thereto and is preferably unitarily formed therefrom. The tab 49 projects upwardly from the plate 26 and is similarly so formed.

The large channel bracket 46 preferably has a rearward extension that passes out of the plate 26, through a hole formed therein, and that receives the nut and bolt assembly 48. The hole in the plate 26 preferably allows considerable travel clearance for the large channel bracket 46 so that it may be precisely positioned by the tightening or loosening of the nut and bolt assembly 48.

The smaller channel brackets 47 are secured to the large bracket 46 by means of one or two screw assemblies 52. When the bracket pair 46-47 encases two pickup head assemblies 50 a single screw assembly 52, positioned between them, is employed. When a single pickup head assembly is captivated then a pair of screw and nut assemblies 52 are employed, one on either side of the pickup head 50.

The plates 24 and 26 may be secured together by means of a pair of horizontal mounting rods 54, (FIG. 2) one on either side of the submodule 18 or 20 at the horizontal central plane therethrough. These rods 54 serve to add structural rigidity to the submodule 18 during assembly.

The bracket pairs 46, 47 serve not only to hold the pickup head assemblies 50 but also to allow those heads to be precisely positioned along a line perpendicular to the radii of the tone wheels 32-37, by adjustment of the screw-nut assemblies 48. The assemblies 50 are also adjustable along the radii by means of an adjustment screw 56 projecting from lthe outer end of each of the assemblies 50.

The screws 56 are each seated in one of four cross channels 58 which are affixed at the upper and lower outer ends of the submodules 18 and 20 and between the plates 24, 26. As best seen in FIG. l each of the channels 58 is preferably affixed in the submodule by having tongue-like extensions 59 at its extreme ends enter conformingly shaped holes in the plates 24, 26.

Referring again to FIG. 2 the pickup head assembly 50 will now be discussed in greater detail. That assembly 50 includes a mounting block 60 to which are affixed a pair of parallel spaced pole pieces 62 which extend to a position just adjacent to the outer edges of the adjacent tone wheels, as wheel 33 in FIG. 2. The pole pieces 62 are made of a low magnetic reluctance material such as iron. Between the wheel edges and the mounting block 60 and betwen the pole pieces 62 is a multi-turn wire induction'coil 64 wound on a core 65 of low magnetic reluctance material. The core 65 spans betwen the pole pieces' 62 and forms therewith part of a magnetic circuit.

In addition the pickup head assembly includes a pair of spaced coil terminals 66 and 67 to which the ends of the induction coil 65 are electrically connected. Insulated tone signal output wires, such as the wires SS and 56, which function to electrically connect the terminals 66 and 67 of each assembly 50 with other components of an electrical musical instrument, are provided connected to those terminals.

A coil spring 69 is also provided about the threaded portion of the screw 56 for bearing against the block 60 and the interior side of the channel 58.

The pole pieces 62 are aixed on the top and bottom of the block 60 by a rivet 61. The rivet 61 as well as the block 60 is preferably made of a non-magnetic material such as nylon or aluminum,

In accordance with an important feature of the present invention the tone wheels 32-37 are, as is best shown in FIGS. 2 and 3, constructed in a laminated manner with a pair of spaced serrated edged discs 70 and 71 with a discshaped plastic magnetic material filling 72 sandwiched between them. The three discs 70, 71 and 72 are mounted on a common hub 73. A spacer ring 74 is positioned between the discs 70 and 71 adjacent to the hub 73.

The discs 70 and 71 are formed from a low magnetic reluctance material such as steel or iron, and the magnetic liller 72 is preferably stamped from a plastic or rubber magnetic sheet. This material is liexible and to a certain extent deformable and comprises a ceramic magnetic material in a rubber or plastic carrier. Because of its deformability the spacer ring 74 is provided to insure sufficient structural rigidity for the wheel 33.

The diameter of the magnetic disc 72 is less than that of the serrated discs 70 and 71 so that at least the serrations thereof project beyond the magnetic disc 72.

In overall operation the above described tone signal generator produces twelve electrical tone signals, six from each submodule. In general operation, the tone signal generator 10 is similar to the tone signal generator described in the aforementioned L. J. Haviland and H. E. Meinema application. That is, the synchronous motor 14 drives the main shaft 28 through the coupling 30 at essentially a constant angular velocity. The shaft 28 in turn drives the wheel shafts 39 and 40` of each submodule at dilering angularly velocities by means of the The general principle of the operation of the tone wheels and pickup head is best illustrated in FIG. 4. Referring to that figure a simplified tone signal generator 100 has: a tone wheel 132 comprising a pair of parallel spaced and serrated edge magnetic material discs 170` and 171 with a disc-shaped magnet 172 sandwiched between them. The wheel 132 is rotated on a shaft 139 in the direction indicated by the arrows 80.

Each of the discs 170 and 172 has an equal number (eight in this case) of serrations about its edge and they are positioned so that the serrations align axially one half cycle out of phase from each other. That is, every crest such as a crest 170C of the serrations of the disc 170 is opposite a trough, such as a trough 171T of the serrations of the disc 171. Similarly, every crest, such as the crest 171C, of the serrations of the disc 171 is opposite a trough, such as the trough 170T, of the serrations of the disc 170.

The magnet or magnetic disc 172 is axially magnetized in the direction and manner indicated by `the double headed arrow 81 and associated conventional symbols (N for north pole and S for south pole).

A simplified pickup head 150 is provided including a non-magnetic support 160 which supports a combined core and pole piece about which is wound an inductance coil 164. The pole piece 162 is a strip of low magnetic reluctance material, such as iron, formed in a generally U-shape (in cross section). 'Ihe longitudinal ends 162A and 162B of the pole piece 162 are approximately as wide as the spacing between the discs 170 and 171 and are .separated from each other a distance equal to one half of the cycle of the serrations formed on the rims of the discs 170, 171.

The pickup head is so positioned that the revolving crests of the serrated discs 170, 171 of the wheel 132 just clear the ends 162A and 162B of the pole piece 162.

With this construction, when the crest of one of the discs or 171 is adjacent to the end 162A the crest of the other one of the discs 171 or 170 is adjacent to the end 162B. Thus at one instance, a crest 170C of disc 170 is juxtaposed to the end 162B of pole piece 162 and a crest 171C of the disc 171 is juxtaposed to the end 162A. As the wheel 132 turns through one half cycle of the serration (one sixteenth revolution of the eight serrated wheel 132) the condition is reversed and a crest 171C of disc 171 is moved into juxtaposition with the pole piece and 162B and simultaneously a crest 170C of disc 170 is moved into juxtaposition with the pole piece end 162A. Half of a serration cycle later the realtionship is again reversed, and so on, continuously, as the Wheel 132 rotates.

Consider now the flux path between the magnetic poles of the permanent disc magnet 172. As the discs 170, 171 are made of a low magnetic reluctance material, such as iron or steel, the vast majority of the magnetic flux will be guided therethrough. In the absence of a pickup head 150, it would be distributed more or less uniformly arching between the serrated edges of the two discs 170, 171.

However, the presence of the low reluctance core and pole piece 162 adjacent to the edge of the wheel 132 presents an lalternative lower reluctance path for magnetic ux. Consequently a significant portion of the magnetic flux is concentrated through the core-pole piece 162.

At an instant in which the wheel 132 is oriented such that the crest 170C is juxtaposed to the end 162A and consequently the crest 171C is juxtaposed to the end 162B, the path of least magnetic reluctance lies through the core pole piece 162 from end 162A to end 162B. That is, a significant portion of the flux from the north poles surface of permanent magnet 172 passes through the disc 160, across the small air gap between the juxtaposed crest 170C and pole piece end 162A through core-pole piece 162 (coil 164) to its other end 162B, across the small air gap between that end and the juxtaposed crest 171C and through the disc 171 to the south pole surface of the magnet disc 172. Note that the magnetic flux passes through the inductance coil 164 from end 162A to end 162B.

When the wheel 132 has turned one half of a serration cycle the opposite disc crest will be adjacent to the pole piece ends 162A and 162B. In this case, the path of least reluctance lies from the north pole surface of the magnetic disc 172 through the disc 170 across the small air gap between the newly juxtaposed crest 170C and the end 162B, through the core-pole piece 162 (coil 164), across the small air gap between end 162A and the juxtaposed crest 171C, and through the disc 171 to the south pole face of the magnetic disc 176. Note that in this case the magnetic flux passes through the inductance coil 164 from end 162B to end 162A, that is, in the opposite direction than before.

Thus, as the wheel 132 rotates, the flux through the core portion surrounded by the coil 164 of the core-pole piece 162 cyclically goes from a maximum in one direction to a maximum in the other direction and then returns to the maximum level in the first direction. For every revolution this cycle is repeated once for every cycle of serrations formed about the edge of the disc 170 and 171. At a midpoint between the two maximums the met liux through the core portion of core-pole piece 162, and thus through the coil 164, is zero.

It is elementary electronics that the instantaneous electric lvoltage developed in an inductance coil is proportional to the number of turns in the coil and the rate of change in the magnetic ux. This relationship can be eX- p pressed in the familiar formula:

dq e-KN dt (l) Where e is the voltage generated, N the number of turns, (p the instantaneous flux passing through the coil, K is a constant of proportionality and d/dt is the mathematical derivative symbol expressing the instantaneous rate of change with respect to time t.

For the coil 164 of the pole piece 150 the instantaneous flux, 1), to at least a first degree of analysis, can be expressed by the following formula:

Where k is equal to the number' of crest on each disc, w is the rotational speed, t is time, and 1; max is given by the following expression:

in which K is a constant of proportionality, M is magnetomotive force, A is the effective area of the crest, and a is the air gap distance between crests and the pole pieces.

The Formula 2 states that the instantaneous flux is equal to the maximum flux cp max multiplied by a function of the number of crests, the angular velocity and time, while the Formula 3 states that the maximum flux is a function of the electromagnetic force, the area of the f crest and the separation of crests.

It follows from Formulas 1, '2 and 3 that NMAkw (4) Where K is a constant of proportionality.

That is, the electrical voltage output of the coil of the simplified tone signal generator of FIG. 4 is proportional to the number of turns, the magnetomotive force, the crest tip area, the number of crests, and the angular rotational speed, and is inversely proportional to the minimum air gap between the pole pieces and the discs, 4and is a function of the number of crests and the angular velocity or rotational speed.

Thus the voltage output will vary cyclically at a frequency determined by the number of crests or serrational cycles and the speed of rotation.

The general analysis and method of operation of the simplified tone wheel 132 and pickup 150 of the simplified generator 100 of FIG. 4 apply to the tone Wheels 32-37 and pickup head assemblies 50 of the tone signal generator of FIGS. 1-3.

As in the case of the tone wheels disclosed in the aforementioned Haviland and Meinema application, it is not necessary to employ serrations of a precisely sinusoidal nature as shown for simplification in FIG. 4. Substantially sinusoidal outputs may be obtained using .gear teeth type serrations such as those shown for the tone wheel 33 in FIGS. 2 and 3. In this case the Formulas 2, 3 and 4 apply `but the letters A represents the tooth tip area, and k the number of teeth per gear-like disc.`

It is not necessary that the ends 62A and 62B of the pole pieces 62 be separated by one-half of the serration cycle about the wheel 32 as any odd multiple of this distance will achieve a similar result.

Thus, again referring to the embodiment of FIGS. l-3 and especially tothe relationship between the pickup head assembly 50 andtone wheel 33 shown in FIG. 2, it can be seen that the pole pieces 62 are there separated by three and a half serration cycles of the serrations on the dises 70 and 71.

The generator 10 of FIGS. l-3 not only develops the twelve fundamental frequencies but does this at high voltage levels, with low harmonic distortion and with low noise, crosstalk, flutter, etc. Furthermore it does it with a maximum of kstandardized parts and assembly, resulting in great economies in manufacture.

Experimental and construction data As an example of the advantages of the present invention the following data of one particular version of the generator 10 is submitted. It should be borne in mind, however, that the present invention may take many other forms that may vary greatly therefrom without departing from the spirit of the invention.

The octave gener-ator 10 produces the octave F6 to E7, notes 66-77, in round numbers, 1398 to 2637 Hz. In use in an organ, the top octave, notes 78-89, can be obtained by frequency doubling; the lower octaves below note 66 would be obtained by dividing. Although the alternator generator 10 can be run at double speed to producey the top octave, it is considered more economical and more desir-able overall to use one octave of doublers than an extra octave of dividers.

The frequency (f) of the tone signal generated in any pickup head assembly S0 is given by the formula:

in which K is a constant of proportionality, R is the rotational speed of the motor 14, r is the gear ratio of the gears 40, 41, 42, 43 that drive the particular tone wheel and k is the number of teeth on each disc of the wheels.

For example, with an 1800 r.p.m. drive motor 14, a gear ratio of 90 to 112 and 58 teeth to the tone wheel 32 (submodule 20) the frequency of the tone signal produced is computed as follows:

1min. 90 MXXOS-ISQSHZ A112 for gears 42 and 41 and 63 to 74 for gears 44 and 43.

The tone wheels of both submodules 18 and 20, 32 and 35 are each preferably fabricated with each disc having 58 teeth, while the wheels 33 and 36 of both submodules 18 and 20 have 73 teeth and the Wheels 34 and 37 of both submodules 18 and 20 each have 92 teeth. The synchronous motor is chosen to operate at 1800 r.p.m. and the following gear ratios are used:

For the shaft 39 a ratio of 92 to 102 was employed and a ratio of 86 to 90 for shaft 40 was employed. That is, the equal sized gears 42 and 41 respectively have 92 and 102 gear teeth while the gears 44 and 43 respectively have 90 and 86.

Under the above construction the assemblies associated with the gears 32-37 in each submodule will substantially produce the notes 66-77, F to E, as follows.

Note: Tone wheel-submodule 66 F 32-20 67 Fil' 35-20 68 G 32--18 69 Gli 35-18 70 A 33-20 71 A14*- 36-20 72 B 33-18 73 C 36-18 74 C# 34-20 75 D 37-20 76 Dii 34-18 77 E 37-18 The theoretical and actual frequencies produced for these notes as well as an expression of the percentage error are given 1n Table II:

TABLE II Frequency Error, Freq. No. Note Theoretical Actual percent 1, 396. 912 960 1, 398. 213 787 O93 1, 479. 978 016 l, 481. 351 349 -l. 093 1, 567 981 536 l, 569. 411 757 091 l, 661. 218 592 1, 662. 666 657 087 1, 760. 000 000 1, 759. 820 800 010 1, 864. 654 432 1, 864. 459 456 010 1, 975. 533 440 1, 975. 294 108 012 2, 093. 004 352 2, 092. 666 654 016 2, 217. 460 672 2, 217. 856 351 -l-. 018 2, 349. 317 824 2, 349. 729 726 018 2, 489. 015 424 2, 489. 411 752 l. 016 2, 637. 020 160 2, 637. 333 318 012 To comply with musical standards it is desirable to have the frequency of A, 440 Hz. as accurate as possible. Therefore in this generator, A7, 1760 Hz. was set up to have the highest accuracy (better than one part in 9000). The accuracy of the others is better than approximately one part in 1100.

From Equation 4 above it can be seen that the output voltage is directly proportional to the number of turns on the coil, the magnetomotive force of the magnet, the area of the tip of the tooth, the rotational speed, and the number of the teeth, and inversely proportional to the gap between the pole pieces and teeth.

IFor a given frequency the number of teeth k and rotational speed w are set. For ease of manufacturing and adjustment a gap a of 10 to 15 mils is considered practical. The number of turns N is also limited since the impedance of the coil must be kept within reasonable limits, e.g., 10K ohms. Therefore, the variable left to produce the required voltage are the magnetomotive force M and the area of the tooth tip A.

IEven these two variables M and A are restricted by certain practical limitations such as magnetic material. This material was Plastiform l-H, and anisotropic elastomer bonded barium-ferrite permanent magnet material which can be machined, cut or punched from at sheets. Ceramic magnets had been considered but proved more expensive since they required grinding and their brittleness is a major disadvantage.

All three tone wheels were fabricated from .045" standard gears and .125 Plastiforms so their overall thickness of .215 allowed the use of the same pole piece width, .406. These dimensions permit the use of assembly tolerances of standard generators.

Measurements with experimental coils indicated that a coil with 5000 turns would produce a satisfactory voltage with pole piece to gear gaps greater than .007". Impedances measured at the operating frequencies of such a coil vary from 12K ohms to 17K ohms.

Experiments using thicker gears, thereby increasing the tooth area A, showed an increase in output voltage. Doubling the gear thickness from .035 to .070 doubles the output voltage. This appears the most desirable method to increase the output voltage at a given pole piece to tooth gap.

The maximum total harmonic distortion that is considered acceptable is The function f(kwt) in Equation 4 determines the waveshape and therefore the resulting harmonic distortion. The function f(kwt) is the reluctance of the magnetic circuit formed by the pickup and tone wheels. As the wheels rotate by the pickup, the reluctance and therefore the flux should vary sinusoidally to produce a sinusoidal voltage. Standard gears having a large number of teeth (the lowest in this generator is 58 teeth) and simple straight pole pieces were found to generate a sinusoidal voltage. The only restrictions were that the pole piece to gear gap should not be too small and that the pole piece separation should be compatible with the tooth spacing.

It was determined experimentally that minimum distortion is achieved when the pole piece thickness approximately equals the tooth tip length. Since the 92 tooth wheel is the most critical, the pole piece thickness was optimized for this 4wheel and used for the 58 and 73 tooth wheels without exceeding the maximum permissible distortion level.

Several early experiments indicated that if the pole pieces span several teeth rather than adjacent teeth, the output is not affected. When the pole pieces are separated by .405 as in the described wheels, they align with the teeth of =all three wheels. This permits the use of one pickup assembly for all three wheels. The gap dimension is most critical for the 92 tooth wheel and the least for the 58 tooth wheel; the former having teeth closer together than the latter. This .405 dimension proved to be suitable from a constructional viewpoint since this is ample space for bobbin with 5000 turns. The pickup assembly shown in FIGURES l and 2 has a cylindrical core which determines the .450 dimension and is held to Within +003.

No differences were observed whether the pole pieces and cores were made from cold rolled steel or such sophisticated metals as Carpenter 49 or Armco iron. All pole pieces and cores were annealed in a dry hydrogen atmosphere.

One method of gaining even higher voltage level output signals from the tone signal generator is shown in FIG. 5. Referring to that figure, a tone Wheel 33 constructed as the wheels 32-37 of the generator 10 of FIGS. 1-3 is mounted for rotation adjacent to a pickup head 250 including a pair of spaced pole pieces 262 with a core 265 mounted therebetween.

About the core 265 is a multiturn inductive coil 264. The pole pieces 262 each have two pickup ends designated 262A, 262A and 262B, 262B. The ends of each pole piece 262 are spaced one tooth cycle apart. The pole pieces 262 are spaced an integral number plus one half of serration cycles apart so pole piece 262 has its ends adjacent to the teeth of one disc (disc 71) and the gap between teeth of the opposite disc (eg. disc 73) while the end of the other pole piece 262 are oppositely positioned.

In this manner a larger amount of magnetic flux s guided through the core 265 to generate an increased voltage therein. The principle of multiple pole piece ends may be extended to have three or even more ends ernployed.

Furthermore, more than one pickup assembly with their coils connected together in phase could be employed with a single tone wheel to produce still larger voltage levels.

As is now obvious a new improved and effective tone signal generator has been described that may yield high level purely alternating electrical tone signals of a high quality. The above described tone signal generator is economical to manufacture and easy to maintain and lends itself to many applications within the musical instrument field.

While particular embodiments of the invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A musical instrument electrical tone signal generator comprising a pair of spaced parallel axially aligned substantially identical tone wheel discs having regularly serrated peripheries, said discs being relatively offset arcuately with the serration humps of one disc opposite the serration valleys of the other, means for permanently magnetizing said discs so that said discs are of opposite magnetic polarity, means for supporting and rotating said discs together in fixed relative position about their common axis, a pickup head comprised of a pair of pole pieces interconnected by an intermediate portion providing a substantially closed magnetic circuit between the pole pieces, each of said pole pieces having a tooth extending parallel to said axis positioned adjacent to and spanning both said disc peripheries, said teeth being spaced apart such that when one tooth is opposite a hump on one disc and a valley on the other the other tooth is opposite a hump on the said other disc and a valley on the said one disc, such that rotation of said discs at a constant speed produces a substantially sinusoidal periodic reversal of magnetic flux through the intermediate portion of the pickup head magnetic circuit, and a pickup coil inductively coupled to the intermediate portion of the pickup head magnetic circuit.

2. A tone signal generator as called for in claim 1 in which the means for permanently'magnetizing said discs comprises a wafer of permanently magnetized material disposed between said discs and having a north pole adjacent one disc and a south pole adjacent the other disc.

3. A tone signal generator as called for in claim 2 in which said wafer is formed of a pliable magnetic material.

4. A tone signal generator as called for in claim 1 in which each of said pole pieces is provided with more than one tooth, and in which all of the teeth of one pole piece are simultaneously aligned with humps on the periphery of one of the discs when all of the teeth of the other pole piece are simultaneously aligned with humps on the periphery of the other disc.

5. A tone signal generator as called for in claim 4 in which the means for permanently magnetizing said discs comprises a wafer of permanently magnetized material disposed between said discs and having a north pole -adjacent one disc and a south pole adjacent the other disc.

6. A tone signal generator as called for in claim 5 in which said wafer is formed of a pliable magnetic material.

7. A musical instrument electrical tone signal generator comprising a pair of spaced parallel axially aligned substantially identical tone wheel discs having regularly serrated peripheries, means for permanently magnetizing said discs so that said discs are of opposite magnetic polarity, means for supporting and rotating said discs together in xed relative position about their common axis, a pickup head comprised of a pair of pole pieces interconnected by an intermediate portion providing a substantially closed magnetic circuit between the pole pieces, each of said pole pieces having a tooth spanning both said disc peripheries, said teeth being spaced apart and oriented .Cil

such that when one tooth is opposite a hump on one disc it is opposite a valley of the other disc and the other tooth is simultaneously opposite a hump on the said other disc and a valley of the said one disc such that rotation of said discs `at a constant speed produces a periodic reversal of magnetic flux through the intermediate portion of the pickup magnetic circuit, and a pickup coil inductively coupled to the intermediate portion of the pickup head magnetic circuit.

8. A tone signal generator as called for in claim 7 in which the means for permanently magnetizing said discs comprises a wafer of permanently magnetized material disposed between said discs and having a north pole adjacent one disc and a south pole adjacent the other disc.

9. A tone signal generator as called for in claim 8 in which said Wafer is formed of a pliable magnetic material.

10. A tone signal generator as called for in claim 7 in which each of said pole pieces is provided with more than one tooth, and in which all of the teeth of one pole piece are simultaneously aligned with humps on the periphery of one of the discs when all of the teeth of the other pole piece are simultaneously aligned with humps on the periphery of the other disc.

11. A tone signal generator as called for in claim 10 in which the means for permanently magnetizing said discs comprises a wafer of permanently magnetized material disposed between said discs and having a north pole adjacent one disc and a south pole adjacent the other disc.

12. A tone signal generator as called for in claim 11 in which said wafer is formed of a pliable magnetic material.

References Cited UNITED STATES PATENTS 1,137,544 4/1915 Severy 310-155 2,552,610 5/1951 White 310-155 3,058,019 10/1962 Eisler 310-156 3,183,387 5/19'65 Wasynczuk 310--263 MILTON O. HIRSHFIELD, Primary Examiner ROBERT SKUDY, Assistant Examiner U.S. C1. X.R. 

